Method of fabricating self-aligned multilevel mask ROM

ABSTRACT

A method of fabricating self-aligned multilevel mask read only memory (ROM). The method can improve the process window to reduce process difficulty by utilizing the self-aligned implantation. Moreover, by utilizing the height difference between different gate polysilicon layers and implantation of the ROM code with self-aligned implatiation to increase the difference between the threshold voltages of different gates, and therefore, multilevel cell transistors with for the mask ROM with multilevel threshold voltages are formed to times the capacity of the mask ROM.

FIELD OF THE INVENTION

The present invention relates to a method of fabricating a mask read only memory (ROM), and more particularly to a method of fabricating a high density mask ROM with multilevel cell transistor by using self-aligned code implantation.

BACKGROUND OF THE INVENTION

Mask ROM is generally made from a number of cell transistors, each serving as a memory unit. When programming is required, ions are implanted into the channel region of selected memory cells so that threshold voltage of these cells is modified. The ‘on’ or ‘off’ state of each memory cell is thus set. In general, a memory cell is created whenever a word line (WL) crosses over a bit line (BL). The memory cell is formed in the word line covered area between two neighboring bit lines. Each memory cell is capable of storing a binary bit of data, either in a logic state of ‘0’ or ‘1’ depending on whether the channel region of the memory cell is implanted or not.

However, when the generation of the mask ROM fabrication is migrated into deep-sub-micro semiconductor process, the higher integration of the integrated circuit, the smaller size of the semiconductor device. While ions implanted into the channel region of selected memory cells, location of the implanting region may have misalignment, so that shifts the threshold voltage of transistors. If location of the implanting region is misaligned, may be shifted to the direction of word line or bit line, this will directly cause data storage error of the ROM cell and disturb the neighboring implanting regions to affect operation property of the memory cell, especially in the shift of word line direction. A conventional way to resolve the problem is using a phase-shift mask (PSM) to program the ROM code. However, this will greatly increase manufacture cost and time so that PSM is not economic.

In addition, ions are implanted just only one time in programming two-level mask ROM so that the cell transistor only has two cases, ion implanted or not. This results each cell transistor of the two-level mask ROM is indicated to have binary signal of “1” or “0”. When memory capacity of two-level mask ROM is required larger, a mount of cell transistors in the two-level mask ROM is increased, so that reducing the integration of the two-level mask ROM, and further reducing yield of the cell transistors.

SUMMARY OF THE INVENTION

Therefore, the present invention provides a method of fabricating a self-aligned multilevel mask read only memory (ROM). The method of the present invention can prevent shift of the threshold voltage causing from misalignment of ion implantation. By utilizing the height difference between different gate polysilicon layers and implanting ROM code to enlarge difference of gate threshold voltage, a multilevel mask ROM is therefore formed to times the memory capacity.

The present invention provides a method of fabricating a mask read only memory (ROM), comprising: forming a gate oxide layer and a polysilicon layer in sequence on a substrate; defining the polysilicon layer; performing a doping process to form a buried bit line in the substrate by using the polysilicon layer as a mask; defining the polysilicon layer to form a plurality of gate layers; forming a plurality of gate spacers around and on the sidewall of the gate layers; forming a silicon oxide layer over the substrate; removing portion of the silicon oxide layer until exposing the top of the polysilicon layer; forming a first patterned photoresist layer on the substrate to cover parts of the polysilicon layer; removing portion the exposed polysilicon layer by using the first patterned photoresist layer as a mask; performing a first ion implantation process to from a first doped region in the substrate under the exposed polysilicon layer by using the first patterned photresist layer as a mask; removing the first patterned photoresist layer; forming a second patterned photoresist layer on the substrate to cover parts of the polysilicon layer; removing portion of the exposed polysilicon layer by using the second patterned photoresist layer as a mask; performing a second ion implantation process to form a second doped region in the substrate under the exposed polysilicon layer by using the second patterned photoresist layer as a mask; removing the second patterned photoresist layer; and forming a conductive layer on the substrate to electrically connect to the exposed polysilicon layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIGS. 1A-1I are schematic, cross-sectional views of illustrating fabrication of the mask ROM of the preferred embodiment in accordance with the present invention;

FIG. 2 is illustrating fabrication of the mask ROM of the preferred embodiment in accordance with the present invention, wherein the cross-sectional line I—I is corresponding to the FIG. 1A;

FIG. 3 is illustrating fabrication of the mask ROM of the preferred embodiment in accordance with the present invention, wherein the cross-sectional line II—II is corresponding to the FIG. 1B;

FIG. 4 is illustrating fabrication of the mask ROM of the preferred embodiment in accordance with the present invention, wherein the cross-sectional line III—III is corresponding to the FIG. 1I; and

FIG. 5 is corresponding to the cross-sectional line IV—IV of the FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1A, a gate oxide layer 102 is formed on a semiconductor substrate 100. A gate conductive layer, such as polysilicon layer 104, is formed on the substrate by chemical vapor deposition (CVD). The illustrated embodiment is described with the polysilicon layer 104 as an example.

Referring to FIG. 2, it is a schematic, top view of the preferred embodiment in accordance with the present invention. FIG. 1A is a cross-sectional view corresponding to the cross-sectional line I—I in the FIG. 2. Referring to FIG. 1A and FIG. 2, the polysilicon layer 104 is defined by conventional photolithography and etch process to form a plurality of parallel, linear polysilicon layer 104, as shown in the polysilicon layer 104 of FIG. 2. A doping process, such as an ion implantation, is performed to dope the substrate 100 and then a plurality of linear doped region 108 are formed in the substrate 100 between the neighbored linear polysilicon layers 104. The doped region 108 is subsequently activized to serve as a buried bit line by annealing process, as shown in FIG. 2.

Referring to FIGS. 1B and 3, FIG. 3 is a schematic, top view of the preferred embodiment in accordance with the present invention, and FIG 1B is a cross-sectional view corresponding to the cross-sectional line I—I in the FIG. 3. The linear polysilicon layer 104 is defined again to form a plurality of gate layers by conventional photolithography and etch process, as shown in the block region for transistors T₁₁˜T₃₃. In FIG. 1B, the gate layers are composed of the polysilicon layer 104 a. A plurality of gate spacers 110 are then formed on the sidewall of the gate layers, and the gate spacers 110 are around the gate layers, as shown in FIG. 3. The gate spacers 110 can be formed with the following exemplary steps. A material of the gate spacers 110, such as silicon oxide, is formed by chemical vapor deposition (CVD) over the substrate 100. Undesired portion of the silicon oxide layer is then removed by anisotropic etching process to leave the gate spacers 110 on the sidewall of the gate layers.

Referring to FIG. 1C, an insulating layer is formed over the substrate 100 and fill the gap between the stacked gate layers. The insulating layer is preferably a high-density plasma (HDP) silicon oxide layer 112 formed by HDP-CVD. Referring to FIG. 1D, a planarization process is carried out to remove undesired portion of the HDP silicon oxide layer 112 until exposing the top of polysilicon layer 104 a. The planarization process is preferably performed with chemical mechanical polishing (CMP), and other process, such as etch back process, also can be used.

Referring to FIG. 1E, a first patterned photoresist layer 114 is formed on the substrate 100 by a sequence of steps, such as coating, exposing and developing, etc. The patterned photoresist layer 114 includes a pattern of first read only memory (ROM) code, and first ROM code is programmed into cell transistors by utilizing the patterned photoresist layer 114. For example, the patterned photoresist layer 114 covers the transistor T₂₂ and exposes the transistors T₂₁ and T₂₃. The exposed portion of the polysilicon layer 104 a is then removed by such as dry etching process by using the first patterned photoresist layer 114 as a mask.

Referring to FIG. 1F, by using the patterned photoresist layer 114 as a mask, an first implantation process is performed to dope the ions into the substrate 100 to form a plurality of doped regions 116 in the substrate 100 underlying the exposed parts of the polysilicon layer 104 a, such as for transistor T₂₁ and T₂₃. Since the ions doped into the substrate 100 can be “self-aligned” so that the patterned photoresist layer 114 serving as a mask can have a larger opening for ROM code, and therefore increase process window of the photolithography process. The problem of shift of threshold voltage causing from mask misalignment can be prevented. Moreover, because the doped region 116 will modify the threshold voltage of the cell transistor, by forming the doped regions 116 for the cell transistors, the threshold voltage of the doped cell transistors will be increased to program first ROM code into cell transistors. The first patterned photoresist layer 114 is then removed.

Referring to FIG. 1G, a second patterned photoresist layer 118 is formed on the substrate 100 by a sequence of steps, such as coating, exposing and developing, etc. The patterned photoresist layer 118 includes a pattern of second ROM code, and second ROM code is programmed into cell transistors by utilizing the patterned photoresist layer 118. For example, the patterned photoresist layer 118 covers the transistor T₂₁ and T₂₃ and exposes the transistors T₂₂.

Referring to FIG. 1H, by using the patterned photoresist layer 116 as a mask, an ion implantation process is performed to dope the ions into the substrate 100 to form a plurality of doped regions 120 in the substrate 100 underlying the exposed parts of the polysilicon layer 104 a, such as for transistor T₂₂. Wherein, the doped dosage for the second doped region 120 is different from the first doped region 116. For example, the doped dosage for the second doped region 120 is greater than the first doped region 116. Since the ions doped into the substrate 100 can be “self-aligned” so that the patterned photoresist layer 116 serving as a mask can have a larger opening for ROM code, and therefore increase process window of the photolithography process. The problem of shift of threshold voltage causing from mask misalignment to result device operation default can be prevented. Moreover, because the doped dosage for the first doped region 116 and second doped region 120 is different, the voltage change of the threshold voltage is different for each cell transistors, so that different level of threshold voltage of the cell transistors are formed. The threshold voltage of the doped cell transistors will be increased to program second ROM code into cell transistors. For example, the transistor T₂₂ has a doped region 120 and the transistor T₂₁ has a doped region 116 and the doped dosage for the second doped region 120 is greater than the first doped region 116, so that the threshold voltage of the transistor T₂₂ will be larger than the transistor T₂₁. While an external voltage between the threshold voltages of the transistors T₂₂ and T₂₁ is applied to the transistors T₂₂ and T₂₁, only the transistor T₂₁ is turned on, so that the transistors T₂₁ and T₂₃ is programmed. By forming multilevel of the threshold voltage, the capacity of the memory is redoubled. The patterned photoresist layer 118 is then removed. Two ROM codes are used as an example. Furthermore, if more ROM codes are needed to program, the steps from FIGS. 1G-1H can be repeated to form desired multilevel mask ROM.

Referring to FIG. 1I, a conductive layer 122 is formed over the substrate 100 and electrically connected to the exposed polysilicon layer 104 a. The conductive layer 122 is the defined by conventional photolithography and etch process to form parallel word lines across the buried bit lines 108, as shown in FIG. 4. Referring to FIG. 5, it is a schematic, cross-sectional view corresponding to the cross-sectional line IV—IV of FIG. 4. The conductive word lines 122 are arranged as shown in FIG. 5. A material of the conductive word lines is preferably made of metal, such as tungsten (W) to reduce electrical resistance and improve the operation speed. A metal silicide layer can be selectively formed to improve adhesion between the polysilicon layer 104 a and the conductive layer 122 in prior to forming the conductive layer 122.

According to above description, the present invention provides a method of fabricating a self-aligned multilevel mask read only memory (ROM). The method of the present invention can prevent shift of the threshold voltage causing from misalignment of ion implantation. By whether connecting between the gate and the word line and implanting ROM code to enlarge difference of gate threshold voltage, a multilevel mask ROM is therefore formed to times the memory capacity.

As is understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are illustrated of the present invention rather than limiting of the present invention. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structure. 

What is claimed is:
 1. A method of fabricating a mask read only memory (ROM), comprising: forming a gate oxide layer and a polysilicon layer in sequence on a substrate; defining the polysilicon layer; performing a doping process to form a buried bit line in the substrate by using the polysilicon layer as a mask; defining the polysilicon layer to form a plurality of gate layers; forming a plurality of gate spacers around and on the sidewall of the gate layers; forming a silicon oxide layer over the substrate; removing a portion of the silicon oxide layer until exposing the top of the polysilicon layer; forming a first patterned photoresist layer on the substrate to cover parts of the polysilicon layer; removing a portion of the exposed polysilicon layer by using the first patterned photoresist layer as a mask; performing a first ion implantation process to form a first doped region in the substrate under the exposed polysilicon layer by using the first patterned photresist layer as a mask; removing the first patterned photoresist layer; forming a second patterned photoresist layer on the substrate to cover parts of the polysilicon layer; removing a portion of the exposed polysilicon layer by using the second patterned photoresist layer as a mask; performing a second ion implantation process to form a second doped region in the substrate under the exposed polysilicon layer by using the second patterned photoresist layer as a mask; removing the second patterned photoresist layer; and forming a conductive layer on the substrate to electrically connect to the exposed polysilicon layer.
 2. The method according to claim 1, wherein a material of the gate spacers comprises silicon oxide.
 3. The method according to claim 1, wherein the silicon oxide layer comprises a high-density plasma (HDP) silicon oxide layer.
 4. The method according to claim 1, wherein removing a portion of the silicon oxide layer until exposing the top of the polysilicon layer comprises using chemical mechanical polishing (CMP).
 5. The method according to claim 1, wherein the first patterned photoresist layer includes a pattern of first ROM code.
 6. The method according to claim 5, wherein the first ROM code is programmed with forming the first doped region.
 7. The method according to claim 1, wherein the second patterned photoresist layer includes a pattern of second ROM code.
 8. The method according to claim 7, wherein the second ROM code is programmed with forming the second doped region.
 9. The method according to claim 1, wherein the first and second ion implantation process is a self-aligned ion implantation process.
 10. The method according to claim 1, wherein a doped dosage of the second doped region is greater than the first doped region.
 11. The method according to claim 1, wherein a material of the conductive layer comprises tungsten.
 12. A method of fabricating a mask read only memory (ROM), comprising: forming a gate oxide layer and a gate conductive layer in sequence on a substrate; defining the gate conductive layer; performing a doping process to form a buried bit line in the substrate by using the gate conductive layer as a mask; defining the gate conductive layer to form a plurality of gate layers; forming a plurality of gate spacers around and on the sidewall of the gate layers; forming an insulating layer over the substrate; removing a portion of the insulating layer until exposing the top of the gate conductive layer; forming a first patterned photoresist layer on the substrate to cover parts of the gate conductive layer; removing a portion of the exposed gate conductive layer by using the first patterned photoresist layer as a mask; performing a first doping process to form a first doped region in the substrate under the exposed gate conductive layer by using the first patterned photoresist layer as a mask; removing the first patterned photoresist layer; forming a second patterned photoresist layer on the substrate to cover parts of the gate conductive layer; removing a portion of the exposed gate conductive layer by using the second patterned photoresist layer as a mask; performing a second doping process to form a second doped region in the substrate under the exposed gate conductive layer by using the second patterned photoresist layer as a mask; removing the second patterned photoresist layer; and forming a conductive layer on the substrate to electrically connect to the exposed gate conductive layer.
 13. The method according to claim 12, wherein a material of the gate conductive layer comprises polysilicon.
 14. The method according to claim 12, wherein a material of the gate spacers comprises silicon oxide.
 15. The method according to claim 12, wherein the insulating layer comprises a high-density plasma (HDP) silicon oxide layer.
 16. The method according to claim 12, wherein removing a portion of the insulating layer until exposing the top of the gate conductive layer comprises using chemical mechanical polishing (CMP).
 17. The method according to claim 12, wherein the first patterned photoresist layer includes a pattern of first ROM code.
 18. The method according to claim 17, wherein the first ROM code is programmed with forming the first doped region.
 19. The method according to claim 12, wherein the second patterned photoresist layer includes a pattern of second ROM code.
 20. The method according to claim 19, wherein the second ROM code is programmed with forming the second doped region.
 21. The method according to claim 12, wherein a doping process is a self-aligned ion implantation process.
 22. The method according to claim 12, wherein the doped dosage of the second doped region is greater than the first doped region.
 23. The method according to claim 12, wherein a material of the conductive layer comprises tungsten. 